1. Field of the Invention
This invention relates to the packaging of opto-electronic devices and arrays, specifically including vertical cavity surface emitting laser arrays and photo detector arrays, that couple to optical fibers.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific device characteristics. In particular, the various material systems can be tailored to emit different wavelengths, such as 1550 nm, 1310 nm, 850 nm, 670 nm, and so on.
VCSELs include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAs) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20, usually having a number of quantum wells, is formed over the lower spacer 18. A p-type graded-index top spacer 22 (another confinement layer) is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type GaAs cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonate at specific wavelengths, the mirror separation is controlled so as to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40, formed by implanting ions (protons), that provides current confinement. Alternatively, the insulating region 40 can be formed using an oxide layer, for example, in accordance with the teachings of U.S. Pat. No. 5,903,588, which is incorporated by reference. In either case, the insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path through the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that it flows through the conductive central opening 42 to the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a common VCSEL structure, and that numerous variations are possible. For example, the doping can be changed (say, by providing a p-type substrate 12), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added.
While individual VCSELs are of great interest, some applications can benefit from arrays of VCSEL elements, while other applications can benefit from arrays of photo detectors. For example, FIG. 2 illustrates a VCSEL array 72 comprised of four, evenly spaced, individual VCSEL elements 73, each of which could be a VCSEL 10 as shown in FIG. 1, and a photo detector array 74 comprised of photo detector elements 75. The individual array elements are usually spaced apart the same distance, for example, 250 microns. Bonding wires 80 connect the array elements to conductors 76 on a substrate that terminate in pads 78.
While generally successful in serial module applications based on a discrete VCSEL and Photo-Detector (PD), applications with VCSEL arrays and photo detector arrays have many more challenges. One particular problem is interconnecting VCSEL arrays and/or photo detector arrays with higher-level systems. Such interconnections often require electrical attachment to another structure (such as a printed circuit board) that provides electrical signals and optical coupling to optical fibers. To assist electrical attachment, VCSEL/photo detector array substrates are usually relatively large, which adds significantly to their cost. Furthermore, the tab bonding that is conventionally used for electrical attachment does not meet the design challenges of high-speed data and telecommunication applications. Such design challenges include well-controlled impedances and low parasitic capacitances.
Many of today's high volume commercial applications of VCSELs involve high data rate (multiple gigabytes per second), serial fiber optic transceiver applications. To provide the required reliability, such applications require hermetically sealed packages. To take advantage of existing investments in device packaging and assembly tooling, it is beneficial to use package configurations that are similar to TO-type hermetic packages for parallel transmitter and receiver components. However, most emerging parallel fiber transceiver implementations for VCSELs use non-standard, non-hermetically sealed packages. This creates problems for system designers, particularly when high reliability in high temperature, high humidity environments is required. Optically coupling arrays to paralleled ribbon optical fiber connectors can be very difficult to do in a package that provides good electrical signal integrity, high optical coupling efficiency, and hermiticity.
Furthermore, optically coupling VCSEL and/or detector array substrates to optical fibers requires precise physical alignment. Such alignment is difficult to achieve, particularly when parallel optical fiber connectors are used.
Therefore, a new technique of packaging semiconductor arrays would be beneficial. Even more beneficial would be a new technique of packaging semiconductor arrays in a hermetically sealed package. Still more beneficial would be a new technique of packaging and hermetically sealing a semiconductor array such that the individual elements of the semiconductor array can be optically coupled to optical fibers. Even more beneficial would be an electrically connectable package that facilitates electrical connections between a hermetically packaged opto-electronic array and external circuitry, while providing for interfacing of individual optical elements with optical fibers.